Exhaust fan assembly

ABSTRACT

An improved exhaust fan assembly is provided in which a fan and damper are mounted in a frame, the damper being mounted at the fan outlet, within an exhaust fan housing structure. Two springs are utilized to hold the damper closed. The damper opens by the force of air exerted during fan operation. During that operation, the springs exert a force which helps to hold the damper open. At least one spring has a top extension that contacts a pin on the damper, keeping the over-center force from becoming so great to hold the damper open permanently. The fan and damper are selfcontained with respect to the rest of the fan housing and the length of the exhaust fan housing can be varied for differing thicknesses of wall through which the housing is to extend.

United States Patent 91 Steiner [45] Feb. 12,1974

[ EXHAUST FAN ASSEMBLY Primary Examiner-William F. ODea AssistantExaminer-Peter D. Ferguson 75 l t R rt E. S S l 1 men or obe miner 1Louis M Attorney, Agent, or Firm-Polster and Polster [73] Assignee:Emerson Electric Co., St. Louis, Mo.

[22] Filed: Feb. 17, 1972 I [57] ABSTRACT [21] Appl. No.: 227,031 Animproved exhaust fan assembly is provided in which a fan and damper aremounted in a frame, the damper being mounted at the fan outlet, withinan exg 98/43 49/ haust fan housing structure. Two springs are utilized"i...- to the ampe ose e dalnper opens the [58] meld of Search 98/4349/197 386 force of air exerted during fan operation. During thatoperation, the springs exert a force which helps to [56] References cuedhold the damper open. At least one spring has a top UNITED STATESPATENTS extension that contacts a pin on the damper, keeping 1,792,5142/1931 Spear 98/43 the over-center force from becoming so great to hold1,528,267 3/1925 Root the damper open permanently. The fan and damperlg; x g are self-contained with respect to the rest of the fan a er h3,120,798 2/1964 Nash 98/43 X ousmg and the length of the exhaust fanhousing can be varied for differing thicknesses of wall through whichthe housing is to extend.

' 11 Claims, 9 Drawing Figures PAIENTED FEB 1 21974 SHEEI 8 BF 3 FIG. 5.

FIG. 4. j

BACKGROUND OF THE INVENTION This invention relates to back draft dampersfor exhaust fans, and in particular, to through-wall exhaust fans. Thoseskilled in the art, however, will recognize the applicability of theconcepts of this invention to other damper applications.

It is common to use exhaust fans in the construction of houses and thelike in a number of areas.

For example, commonly a new home has a throughwall exhaust fan near thecooking area of the home to dissipate the heat and odors generated infood preparation. Conventionally, the exhaust units have a fan mountedin a housing. the housing extends through the wall structure of thehouse and the blower or fan is adapted to blow air through the housingto the external atmosphere. Because these units are vented to theoutside atmosphere, it is common to utilize some form of back draftdamper in conjunction with the units to prevent rain, cold air, insectsand the like from getting in. Conventionally, the damper has beeninstalled on the external side of the through-wall unit.

While these units work well for their intended purposes, they exhibitinherent disadvantages heretofore unresolved in the prior art. Prior artdampers conventionally are constructed from sheet metal and are attachedto a sheet metal enclosure. Such construction presents a highlyobjectionable noise problem as the back draft damper tends to close withsome force. It is conventional to line the damper-enclosure contact areawith some form of padding, conventionally of the felt type, to lessenthe metal chatter. Unfortunately, it is difficult to attach the paddingto the damper and have it remain there for any extended period of time.It also generally is difficult to position the padding properly.

While recent improvements in damper constructions, for example, thatdisclosed in the US Pat. to Steiner No. 3,589,268, issued June 29, 1971,alleviated cerain objectional features of the stamped metal damper byutilizing a non-metallic material for the damper, the use ofnon-metallic material for damper construction in certain applications,and in particular, in through-wall applications, has been heretoforeunfeasible because of the conventional placement of that damper on theoutside, along the outlet of the exhaust fan structure, re-

mounting procedure is simplified by utilizing a flanged opening at theunit impeller and by designing the damper to seal about the flange.

One of the objects of this invention is to provide an exhaust unithaving a back draft damper mounted adjacent the impeller of the blower.

mote from the impeller of the blower unit. In some through-wall units,this means that the damper 'is exposed to the ravages of theenvironment, and not all non-metallic dampers will stand up under theconditions of extreme heat and cold to which they are liable to beexposed. In addition, the through-wall unit, to be economically feasiblein a large scale production, must be adaptable to a variety of wallthicknesses. Heretofore, no reliable non-metallic unit was consideredcommercially feasible because the adjustability requirement for thehousing was believed to preclude mounting of the damper at any pointexcept near the outlet of the exhaust unit.

My invention permits use of a non-metallic back draft damper by mountingthe damper adjacent the unit impeller. A unique system of back biasingthe damper is provided which enables the damper to close regardless ofthe thickness-of the wall, hence the length of the housing, therebyretaining the necessary adjustability features provided by oldersystems. The damper Another object of this invention is to provide sucha through-wall exhaust unit having a nonmetallic back draft damper.

Yet another object of this invention is to provide novel means for backbiasing a back draft damper.

Still another object of this invention is to provide a dampercharacterized by silent operation.

Yet another object of this invention is to provide increased air flow inan exhaust unit by utilizing louvers in the external side of athrough-wall exhaust system.

Other objects will be apparent to those skilled in the art in light ofthe following description and accompanying drawings.

SUMMARY OF THE INVENTION In accordance with this invention, generallystated, a through-wall exhaust unit is provided having a nonmetallicback draft damper mounted adjacent the impeller of the blower. Thedamper is back biased by a spring arrangement which prevents the damperfrom locking itself in an open position.

In the preferred embodiment, the damper is mounted in a housing which isadjustable to correspond to a variety of wall thicknesses. The externalportion of the housing is louvered to increase exhaust fan (blower)output without requiring increased blower capacity.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIG. 1 is a view inperspective of one illustrative embodiment of exhaust fan unit of thisinvention;

FIG. 2 is a view in front elevation of a self-contained fan-damperassembly of the unit shown in FIG. 1; I

FIG. 3 is a view in rear elevation taken along the line 3-3 of FIG. 1;

FIG.4 is a view in side elevation taken along the line 4-4 of FIG. 3;

FIG. 5 is a top plan view of the assembly shown in FIGS. 2-4;

FIG. 6 is an enlarged plan view of a bumper unit utilized in conjunctionwith the damper shown in its mounted position in FIG. 2; a

FIG. 7 isa view in side elevation taken along the line 7-7 of FIG. 6;

FIG. 8 is an enlarged view in side elevation of a spring utilized toback bias the damper of FIG. 2; and

FIG. 9 is a view in side elevation taken along the line 9-9 of FIG. 8.

DESCRIPTION OF THE PREFERRED I EMBODIMENT and includes a stator assembly5 having a plurality of windings 6 associated with it in accordance withconventional motor design theory. In the embodiment il- Blower 2includes a motor 8. Motor 8 is conventional lustrated, the windings 6are enclosed by a shroud l4. Stator assembly has an axial openingthrough it in which a rotor assembly, not shown, is rotatably mounted. Arotor assembly includes a shaft operatively connected to it, likewisenot shown, on one end of which is mounted an impeller 7. Motor 8 isattached to a bracket 9. Bracket 9 is made of sheet metal, rectangularin plan, U-shaped in side elevation, with integral mounting tabs at eachend, and a central mounting portion 12. The bracket is blanked outbetween the tabs 15 and the central mounting portion 12 and turned up atthe edges to form flanged rails 11. Portion .12 has a square opening 16in it, through which a cover attaching bolt 13 is placed. Bolt 13, inthe embodiment illustrated, is nothing more than a conventional, flatheaded bolt with a square shank. The motor 8, when mounted to thebracket 9 locks the bolt 13 within the opening 16.

Motor 8 is attached to the central portion 12 of bracket 9 by aconventional nut-bolt combination l0.

Mounting tabs 15 have an opening 17 in them, through which fasteners arerun to attach the motor bracket to the enclosure 3. In the particularembodiment illustrated, an annular vortex ring 18 is connected to thebracket by spotwelding or the like along the tabs 15. Ring 18 has atransverse lower wall 19 and a vortex wall 20, tending axially withrespect to the axis of rotation of the fan. The purpose and function ofring 18 is described in detail hereinafter.

Motor 8 is electrically connected to a cord 21 with two conductors in itand a male plug connected to them.

In the embodiment illustrated, enclosure 3 comprises a motor mountingframe 22, telescoping shell 23 and an external portion 24.

Motor mounting frame 22, in the embodiment illustrated, is substantiallysquare and has a top 25, a bottom 26, and side walls 27 and 28. Motormounting frame 22 has a open back 29 and a front wall 30. Front wall 30has a circular opening 31 through it defined by an annular flange 32which projects outwardly with respect to the motor 8 and isperpendicular to the front wall 30. When the motor 8 is mounted withinmotor mounting frame 22, the wall 19 of the ring 18 abuts the back ofthe front wall 30 while the wall is adjacent but spaced from the annularflange 32. This arrangement permits effective sealing of the unit toback drafts, as later explained.

The front wall 30 has mounting means associated with it for attachingthe bracket 9 to it. Common screws through the opening 17 work well. Aconnection box 33, with a female socket mounted in it to receive themale plug on the motor cord may be utilized to connect the electricalconnections 21 for the motor 8 to the external source of power notshown. In FIG. 1, the connection box 33 is shown as mounted in the lowerright hand corner, to show the female socket. In FIG. 3, it is shown asmounted in the upper left hand corner. The construction of the frame 22permits mounting either place. The top and the bottom 26 have a pair ofparallel slits 34 in them. The slits 34 provide a portion of theadjustable feature for the exhaust unit 1 as later explained in detail.

Referring now to FIGS. 2, 4 and 5, it is observed that a pair of legs 35extend perpendicularly to the front wall from opposite ends of the top25. The legs are parallel to one another and aligned, and have laterallyaligned openings defined by integrally formed journals 48. In theembodiment of FIG. 4, a second pair of legs extend from the wall 30 nearthe bottom 26. The legs near the bottom 26 serve no function, but areshown because the top and bottom are identical and are obtained in asingle punching operation. For the functioning of my invention, however,only one pair of legs 35, or their equivalent, need be provided. Whilethe particular embodiment illustrated has the legs 35 formed integrallywith the top-bottom members of motor mounting section 22, those skilledin the art will recognize that the legs 35 may be formed from or with afront wall 30 or individually formed and later attached to the section22. The front wall 30 has a pair of T-shaped openings 36 in it, alongeach side of the annular flange 32.

Damper assembly 4 comprises a damper 37 and a pair of mounting springs38. Damper 37 is constructed of a vibration absorbing, syntheticmaterial and includes a front 38, a side wall 39 and a back 40. The sidewall 39 extends about the perimeter of the damper 37 and isperpendicular to the front 37 and back 40, extending rearwardly, towardthe motor 8, from the back 40. Side wall 39 has a pair of oppositelydisposed, integral bosses 42 from which pivot pins 41 project, onopposite sides of the damper. The pivot pins 41 are aligned on an axisalong a top edge 43 of the side wall 39.

Spaced below the bosses 42, on both sides of the damper, spring anchors44 project from the side wall 39. They are aligned transversely of thedamper and are offset rearwardly with respect to the axis of the pivotpins 41 when the damper is closed, as shown in FIG. 4. Spring anchors 44are formed integrally with the wall 39. In the embodiment shown, theyare cylindrical and have an annular channel in them as may best be seenin FIG. 2.

Side wall 39 also has a spring stop 45 extending outwardly from it onone side of the damper. Spring stop 45 is a cylindrical section formedintegrally with side wall 39.

Side wall 39 has a plurality of fastening bosses 46 formed integrallywith and extending outwardly from it. Fastening bosses 46, in theembodiment illustrated, are made up of facing angles defining aninverted T-shaped channel. Fastening bosses 46 are intended to receive aresilient stop 47, described in detail hereinafter.

The pivot pins 41 are rotatably mounted in the pair of fixed journals 48formed in each leg 35. Placement of pivot pins 41 within journals 48pivotally mounts damper 37 at the opening 31. The side 39 is adaptedclosely but freely to surround annular flange 32 while resilient stops47 are sized to abut front wall 30 in the closed position of the damper37 illustrated in FIG. 4.

Spring 38 is a broad band, serpentine design, best illustrated in FIGS.8, 2 and 4. As can be seen by comparing those Figures, the spring isinverted in FIG. 8, with respect to its orientation in use. Spring 38has a free outer end 49, an attaching crook 51, a compression loop 50,and an inner mounted end 52. The end 52, in the embodiment illustrated,has notches 53 in it to define a T-shaped end. As is best illustrated inFIGS. 2 and 4, one spring 38 is frictionally mounted to each springanchor 44 by snapping the attaching crook 51 over it. The spring 38 onthe side of the damper from which-the spring stop 45 projects is sopositioned that the end 49 abuts spring stop 45 when the damper isopened. This abutment is important in the operation of the damper, as isexplained in detail hereinafter. Although only one spring stop 45 isillustrated in the drawings, those skilled in the art will recognizethat two can be provided and both of the springs will be so positioned.

The inner end 52 of each spring is inserted in the opening 36 againstthe bias of the spring, and the notches 53 permit the end to go into thestem of the T, with the cross of the T of the end 52 behind the frontwall 30. The downward bias of the spring keeps it in place.

Resilient stop 47 is thin and flat, and dart shaped in plan, as bestillustrated in FlGS. 6 and 7. As there shown, stop 47 has a wedgeshapedblunt end 54 and a base 55 joined by a reduced part 56. The part 56 iscaged within the channel of the fastening bosses 46. End 54 strikes thefront wall 30 when the damper is closed.

Telescoping shell 23 is square in front elevation and includes a top 57,a side wall 58 and 59, and a bottom 60. Top 57 and bottom 60 have a pairof openings 61 for receiving conventional metal screws 62. The openings61 are positioned so as to align with the slits 34 of the motor mountingframe 22. The opening defined by the walls of the telescoping shell 23is large enough to permit insertion of motor mounting frame 22.

External portion 24 includes a rectangular frame 63, a flange part 64and a pyramidal section 65.

Rectangular frame 63 has four walls, a top 66, sides 67 and 68 and abottom 69. Top 66 and bottom 67 have a pair of slits 70 in them. Theslits 70 are parallel and designed to align with at least a pair ofopenings 61 in top 57 and bottom 60 of telescoping shell 23.

Flange 64 extends outwardly about the perimeter of the frame 63 and maybe formed integrally with either frame 63 or section 65, or may beconstructed separately and attached to either part. In the embodimentillustrated, the flange 64 is formed integrally with the,

section 65 and the frame 63 is flared along an end 70 for attachmenttherewith. Section 65 includes a front wall 71 and a first and a secondside 72 and 73 respectively. The sides 72 and 73 have a plurality oflouvers 74 lanced in them. The pyramidal section 65 is open bottomed.

As thus described, enclosure 3 comprises three sheet metal units whichmay be varied in length to correspond to a plurality of wallthicknesses. That is, motor mounting frame 22, telescoping shell 23, andexternal portion 24 are telescopically mounted to one another and may beadjusted for a variety of wall thicknesses ranging, in the embodimentillustrated, between four and ten inches. The various parts are adjustedalong the slots 34 and slots 70 by loosening the middle screws 61 and 75along both the upper and lower portions of the enclosure 3. Onceinstalled, the flange 64 abuts the external wall covering and thepyramidal section 65 extends outwardly therefrom. Motor mounting frame22 is positioned so that the back 29 opens within the building. Back 29may be closed by a decorative grill which also serves as an inlet to theblower 2. The outlet of the blower 2 is the opening 31 in the front wall30 of motor mounting section 22. As previously indicated, it is at lnoperation, air is forced against damper 37 by the impeller 7 whichcauses the damper 37 to swing out, around the axis of the pins 41. Asthe damper 37 opens, the spring anchor 44 swings outwardly with respectto the plane of the pivot pins 41 and the springs 38 would tend to exerta force to hold the damper open were it not for the abutment of thespring stop 45 and the end 49 of the spring. The damper is preventedfrom being biased open permanently under the influence of the springs bythe restraining action of end 49 of the spring. The end 49 and stop 45not only prevent the damper and spring from over-centering completelybut resiliently bias the damper toward closing when the fan is turnedoff. Thus, the spring keeps the damper firmly closed when the fan isoff, and tends to counter balance the damper to permit it to be andremain opened wide by the fan, the amount of opening being regulated bythe end 49. The resilient stops 47 cushion the closing material, therebypermitting extremely quiet damper that location that the damper 37 ismounted. The

damper 37 will always be so mounted regardless of the position of theframe and housing elements of the enclosure 3.

operation. Removal of the damper from attachment with external unit 24,permits use of the louvers 74 in the sides 72. Dampers in the prior artgenerally were arranged to close the open bottom of the pyramidalsection 65. Louvers in the sides of the section 65 were then impracticalbecause their use defeated the purpose of the damper. With the placementof the damper as described above, louvers can in fact be used. Such usegreatly increases the efficiency of the blower 2 by relieving pressureon the impeller 7.

Numerous variations, within the scope of the appended claims, will beapparent to those skilled in the art in light of the foregoingdescription and accompanying drawings. Thus, various parts described asintegrally constructed may be constructed separately and later attached.The shape of the enclosure 3 may be varied. Thus, while I find itconvenient to utilize three elements for the enclosure 3, other elementcombinations may be used. External portion 24 may be altered. While Ifind the'use of louvers 74 extremely effective, other embodiments of myinvention may eliminate them, alter their size or shape or change theirpositioning. Other forms of resilient stops 47 may be used in place ofstops shown or in conjunction with them. For example, conventional feltpadding may, be utilized along the interior portion of side wall 39 andback 40 of damper 37 in place of or in conjunction with the resilientstops 47 illustrated in the drawings and described above. I prefer theuse of the stops 47 because of the difficulties in placing andmaintaining the position of the felt and the hazards inherent in the useof felt. The design and configuration of the springs 38 may be varied.These variations are merely illustrative.

Having thus described the invention, what is claimed and desired to besecured by Letters Patent is:

1. An exhaust fan unit comprising:

a housing including an external section and a blower mounting frameadjustably mounted with respect to said external section, said mountingframe having an inlet side and an outlet side;

a blower mounted to said mounting frame so as to direct a volume of airfrom said inlet side to said outlet side;

a back draft damper mounted to the outlet side of said mounting frame,said damper being mounted for pivotal movement with respect to saidframe, about a pivot axis, between closed and opened positions;

a spring stop connected to said damper;

a spring anchor mounted to said damper, said spring anchor being offset,toward the inlet side, from said pivot axis when said damper is closed;and

at least one spring element attached to said spring anchor having aserpentine contour between a first end and a second end, one of saidfirst and said second ends of said spring element being attached to saidhousing and the other of said first and said second ends being free andprojecting towards the outlet side, the said free end and said springstop being positioned to abut when the damper is opened, therebylimiting the opening of the damper.

2. The fan unit of claim 1 wherein the opening of the damper moves thespring anchor to a point just short of over-centering, whereby theeffect of the bias of the spring is reduced in the damper-open position.

3. A through wall exhaust fan unit comprising:

a housing, said housing including a blower mounting frame having aninlet side and an oulet side;

a blower mounted to said blower mounting frame so as to direct a volumeof air from said inlet side to said outlet side;

a back draft damper mounted to the outlet side of said mounting frame,said damper being mounted for pivotal movement with respect to saidframe, about a pivot axis, between closed and opened positions;

a spring anchor mounted to said damper, said spring anchor being offset,toward the inlet side, from said pivot axis when said damper is closed;

at least one spring element having a first end and a second end, one ofsaid first and said second ends being attached to said blower mountingframe and the other of said first and said second ends being free andprojecting toward the outlet side, said spring element being attached tosaid spring anchor between said first and said second ends; and

a spring stop mounted to said damper, said free end of said spring andsaid spring stop being arranged to abut in at least one open position ofsaid damper to limit opening of said damper.

4. The fan unit of claim 3 wherein said housing is further characterizedby an external portion and a shell portion, said blower mounting frame,said shell portion and said external portion being adjustably mounted toone another, said external portion having a pyramidal silhouette havingthree walls, at least one of said walls having a plurality of louveredopenings in them.

5. in an exhaust fan unit having a housing, said housing including ablower mounting frame having an inlet side and an outlet side, theimprovement which comprises a back draft damper pivotally mounted tosaid blower mounting frame at said outlet side and adapted for rotationwith respect to said housing about a pivot axis so as to define at leasta closed position, a plurality of opened positions, and an outermostopen position, said damper comprising a structure having a pair ofpivots and a pair of spring anchors attached to it, one each of saidpivots and said spring anchors projecting outwardly on opposite sides ofsaid damper along parallel but laterally displaced axis, and at leastone spring element having a first end and a second end, said first endbeing operatively connected between said housing and said spring anchorso as to exert a force tending to open said damper in said plurality ofopen positions of said damper, said second end of said spring coactingwith said spring stop to enable said spring to exert a force tending toclose said damper in said outermost open position of said damper.

6. The improvement of claim 5 wherein said pivot pair, said springanchor pair, and said spring stop are integrally formed with saiddamper.

7. The improvement of claim 6 wherein said outlet comprises a wallhaving an opening in it for defining said outlet side, said openinghaving a flange extending about its perimeter, said damper comprising aflange engaging portion adapted to seal said outlet opening in a closedposition of said damper.

8. In an exhaust fan unit housing having an inlet side and an outletside, a blower mounted in said housing, the improvement which comprisesa back draft damper pivotally mounted to said housing adjacent saidblower, said damper being adapted for rotation with respect to saidhousing between at least closed and opened positions, said dampercomprising a resilient material structure having a pair of pivotsprojecting oppositely outwardly from said damper along a pivot axis, atleast one anchor means projecting outwardly from said damper along asecond axis, said second axis being parallel to but laterally spacedfrom said pivot axis, at least one spring operatively connected betweensaid anchor means and said housing, and a spring stop mounted to saiddamper, said spring stop and said spring coacting to exert a closingforce on said damper in said opened position.

9. The improvement of claim 8 wherein said outlet side comprises a wallhaving an opening in it for defining said outlet side, said openinghaving a flange extending about its perimeter, said damper comprising aflange engaging portion adapted to seal said outlet opening in a closedposition of said damper.

10. In an exhaust fan unit having a housing and a back draft damperrotatably mounted between a closed position, a plurality of openpositions and an outermost open position, the improvement whichcomprises means for exerting a force tending to open said damper in saidplurality of open positions of said damper, and means for exerting aforce tending to close said damper in an outermost open position of saiddamper, said open force exerting means including a spring elementoperatively connected between said housing and said damper, said closedforce exerting means including a spring stop attached to said damperwhich coacts with said spring in at least the outermost open position ofsaid damper to enable said spring element to exert a force on saiddamper in a direction of the closed position of said damper.

11. An exhaust fan unit comprising:

a housing selectively adjustable between a minimum housing length and amaximum housing length including an external section and a blowermounting frame adjustably mounted with respect to said external section,said mounting frame having an inlet side, a wall having an opening in itattached to said mounting frame, said opening defining an outlet sidefor said mounting frame;

a blower mounted to said mounting frame so as to dipositions of saiddamper;

rect the volume of air from said inlet side to said at least one springelement operatively connected beoutlet side; tween said housing and saiddamper, said spring ela back draft damper mounted to the outlet side ofement havingafree end extending outboard of said said mounting frame,said damper being mounted wall in the direction of air volume flow forall posifor pivotal movement with respect to said mounttions of saiddamper; and ing frame between a closed position blocking said means forengaging the free end of said spring eleopening in said wall and an openposition, said ment to exert a closing force on said damper in itsdamper being mounted so as to be surrounded by closed position. saidhousing at all lengths of said housing and at all UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3,791 ,281 Dated February12, 1974 Inventor(s) Robert E te1ner It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 16, after "housing.", "the" should-read the line 40,"cerain" should read certain Column 4, line 20, after "a front" thereference numeral "38" should read 90 line 22, "the front 37" shouldread the front 90 Column 9, line 2, "the" should read I a- Column 10,line 9, "closed" should read open I DRAWINGS V Figure 2, the referencenumeral "38", shown in the center of the drawing figure, should read 90Signed and sealed this 24th day of September 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PO-IOSO (10-69) USCOMM-DC 60376-P69 I v.5. GOVERNMENTPRINTING OFFICE: 930

1. An exhaust fan unit comprising: a housing including an externalsection and a blower mounting frame adjustably mounted with respect tosaid external section, said mounting frame having an inlet side and anoutlet side; a blower mounted to said mounting frame so as to direct avolume of air from said inlet side to said outlet side; a back draftdamper mounted to the outlet side of said mounting frame, said damperbeing mounted for pivotal movement with respect to said frame, about apivot axis, between closed and opened positions; a spring stop connectedto said damper; a spring anchor mounted to said damper, said springanchor being offset, toward the inlet side, from said pivot axis whensaid damper is closed; and at least one spring element attached to saidspring anchor having a serpentine contour between a first end and asecond end, one of said first and said second ends of said springelement being attached to said housing and the other of said first andsaid second ends being free and projecting towards the outlet side, thesaid free end and said spring stop being positioned to abut when thedamper is opened, thereby limiting the opening of the damper.
 2. The fanunit of claim 1 wherein the opening of the damper moves the springanchor to a point just short of over-centering, whereby the effect ofthe bias of the spring is reduced in the damper-open position.
 3. Athrough wall exhaust fan unit comprising: a housing, said housingincluding a blower mounting frame having an inlet side and an ouletside; a blower mounted to said blower mounting frame so as to direct avolume of air from said inlet side to said outlet side; a back draftdamper mounted to the outlet side of said mounting frame, said damperbeing mounted for pivotal movement with respect to said frame, about apivot axis, between closed and opened positions; a spring anchor mountedto said damper, said spring anchor being offset, toward the inlet side,from said pivot axis when said damper is closed; at least one springelement having a first end and a second end, one of said first and saidsecond ends being attached to said blower mounting frame and the otherof said first and said second ends being free and projecting toward theoutlet side, said spring element being attached to said spring anchorbetween said first and said second ends; and a spring stop mounted tosaid damper, said free end of said spring and said spring stop beingarranged to abut in at least one open position of said damper to limitopening of said damper.
 4. The fan unit of claim 3 wherein said housingis further characterized by an external portion and a shell portion,said blower mounting frame, said shell portion and said external portionbeing adjustably mounted to one another, said external portion having apyramidal silhouette having three walls, aT least one of said wallshaving a plurality of louvered openings in them.
 5. In an exhaust fanunit having a housing, said housing including a blower mounting framehaving an inlet side and an outlet side, the improvement which comprisesa back draft damper pivotally mounted to said blower mounting frame atsaid outlet side and adapted for rotation with respect to said housingabout a pivot axis so as to define at least a closed position, aplurality of opened positions, and an outermost open position, saiddamper comprising a structure having a pair of pivots and a pair ofspring anchors attached to it, one each of said pivots and said springanchors projecting outwardly on opposite sides of said damper alongparallel but laterally displaced axis, and at least one spring elementhaving a first end and a second end, said first end being operativelyconnected between said housing and said spring anchor so as to exert aforce tending to open said damper in said plurality of open positions ofsaid damper, said second end of said spring coacting with said springstop to enable said spring to exert a force tending to close said damperin said outermost open position of said damper.
 6. The improvement ofclaim 5 wherein said pivot pair, said spring anchor pair, and saidspring stop are integrally formed with said damper.
 7. The improvementof claim 6 wherein said outlet comprises a wall having an opening in itfor defining said outlet side, said opening having a flange extendingabout its perimeter, said damper comprising a flange engaging portionadapted to seal said outlet opening in a closed position of said damper.8. In an exhaust fan unit housing having an inlet side and an outletside, a blower mounted in said housing, the improvement which comprisesa back draft damper pivotally mounted to said housing adjacent saidblower, said damper being adapted for rotation with respect to saidhousing between at least closed and opened positions, said dampercomprising a resilient material structure having a pair of pivotsprojecting oppositely outwardly from said damper along a pivot axis, atleast one anchor means projecting outwardly from said damper along asecond axis, said second axis being parallel to but laterally spacedfrom said pivot axis, at least one spring operatively connected betweensaid anchor means and said housing, and a spring stop mounted to saiddamper, said spring stop and said spring coacting to exert a closingforce on said damper in said opened position.
 9. The improvement ofclaim 8 wherein said outlet side comprises a wall having an opening init for defining said outlet side, said opening having a flange extendingabout its perimeter, said damper comprising a flange engaging portionadapted to seal said outlet opening in a closed position of said damper.10. In an exhaust fan unit having a housing and a back draft damperrotatably mounted between a closed position, a plurality of openpositions and an outermost open position, the improvement whichcomprises means for exerting a force tending to open said damper in saidplurality of open positions of said damper, and means for exerting aforce tending to close said damper in an outermost open position of saiddamper, said open force exerting means including a spring elementoperatively connected between said housing and said damper, said closedforce exerting means including a spring stop attached to said damperwhich coacts with said spring in at least the outermost open position ofsaid damper to enable said spring element to exert a force on saiddamper in a direction of the closed position of said damper.
 11. Anexhaust fan unit comprising: a housing selectively adjustable between aminimum housing length and a maximum housing length including anexternal section and a blower mounting frame adjustably mounted withrespect to said external section, said mounting frame having an inletside, a wall having an opening in it attached to said mounting frame,said opening defining an outlet side for said mounting frame; a blowermounted to said mounting frame so as to direct the volume of air fromsaid inlet side to said outlet side; a back draft damper mounted to theoutlet side of said mounting frame, said damper being mounted forpivotal movement with respect to said mounting frame between a closedposition blocking said opening in said wall and an open position, saiddamper being mounted so as to be surrounded by said housing at alllengths of said housing and at all positions of said damper; at leastone spring element operatively connected between said housing and saiddamper, said spring element having a free end extending outboard of saidwall in the direction of air volume flow for all positions of saiddamper; and means for engaging the free end of said spring element toexert a closing force on said damper in its closed position.